1. Technical Field
The present invention generally relates to a method of etching an oxide layer and a nitride layer, and more particularly to a method of forming an oxide-nitride-oxide (ONO) structure.
2. Description of the Related Art
Usually, there are many applications of nitride and oxide within integrated circuits (ICs), for example, an isolation structure between adjacent transistors, a gate spacer, an etch stop layer, a protection layer for outermost layer of IC chip, an ONO structure, etc.
A non-volatile static random access memory (nvSRAM) is taken as an example. The nvSRAM generally includes a static random access unit and a non-volatile memory unit. The static random access unit is applied for temporarily accessing data when a power is supplied. The non-volatile memory unit is capable of storing data even when the power supply to it is cut off. The nvSRAM uses a silicon-oxide-nitride-oxide-silicon (SONOS) structure formed therein as a memory cell. During the operation of the nvSRAM, data signals (e.g., digital signals “0” and “1”) implement action such as write (programming), erase or read in the SONOS structure.
However, during fabrication process of the SONOS structure, it is found that sidewalls of a shallow trench isolation (STI) structure that protrude out of a surface of wafer usually have unwanted residual material layer forming a redundant fence (termed as sidewall residual or redundant spacer). Such redundant fence would change a surface profile of the STI structure and increase a width of the STI structure. Moreover, the redundant fence would serve a mask of subsequent etching processes and implantation processes. The influence caused by the redundant fence can not be eliminated by adjusting the position of the implantation mask or the direction of the implantation process. As a result, the area of doped regions and the etching window are decreased. In other words, the redundant fence would cause reduction in the effective area of active region, especially for a narrow width device. Even more, the redundant fence would cause decrease in current of the narrow width device and thus adversely influence the operation of the narrow width device.
If a process time of the etching process is prolonged, an additional isotropic etching process is required, or an additional anisotropic etching process is required in order to remove the redundant fence, which may seriously damage the other material layers or components on the surface of wafer. For example, a serious side effect may therefore damage the SONOS structure. The fabrication and operation of the device may be worse due to such an additional etching process.